Telephone crafts-persons such as splicers, often have to handle in-service telephone wires bare-handed. While the telephone network operates at normal voltages well below values that could be human-hazardous, there remains the possibility of unintended application of foreign voltages high enough in magnitude to jeopardize these crafts-people.
One potential source of such voltages is the terminal equipment itself, which occasionally include circuits connected to commercial power. The design, manufacture, installation, maintenance and repair of telephone terminal equipment by a single responsible entity has in the past assured that commercial voltages present in the terminal do not appear on the telephone line. However, with the recent expansion of the direct connection of customer (as opposed to telephone company) owned and maintained equipment, the power to protect crafts-persons from hazardous voltages is no longer primarily that of the telephone company. The risks which this situation poses to telephone crafts-persons, and on a different plane, to central office equipment, is recognized by industry groups and regulators.
Devices have been employed in the past to provide a protective barrier for the telephone network against several types of harms that can originate in customer-provided equipment. Few of these, however, have provided solely hazardous voltage protection. Additionally, circuit requirements for such devices in the past have not been adequately predicated upon the nature of the hazard itself. Studies have revealed that the main factors affecting voltage shock severity are: the level and duration of current applied through a human body; the current path through the body; and the individual's body weight. The critical concern thus is electrical current.
Additionally, a majority of telephone lines operate with balanced, non-grounded terminations; and with metallic signaling. An effective method of detecting and protecting against the application of hazardous voltages originating in terminal equipment must take these conditions into account as well.